Katy Perry Tweets Picture of John Mayer in Santa Costume

Ho ho ho! John Mayer is so much more than a cad with a big mouth when it comes to relationships. The singer-songwriter and guitarist is also a great role player.

His GF Katy Perry tweeted a picture of him in a Santa Claus costume -- giving off a sly grin in place of the furry white beard -- on Christmas.

Perry added "Santa Baby" as the caption, which also affirms their status as a couple. You don't call a guy "baby," Santa outfit or not, unless things are romantic! It's a foregone conclusion at this point.

Mayer and Perry reportedly spent the holiday with her family in Santa Barbara, Calif. Word is that Mayer gave Perry a customized guitar as a present, as well. How thoughtful of him!

Us Weekly reports that the musical twosome is "seriously in love and exclusive" and that Mayer loves how "cool and smart she is." The source also said that Mayer is determined to shed his previous rep as a serial dater who blabs about his relationships. (He once referred to Jessica Simpson as "sexual napalm.")

But that's all Mayer of old. The insider said, "John has proved he's matured and learned from his mistakes . . . Katy is blown away."

The duo also visited a local hospital on Dec. 22, making the charitable rounds for the holiday.

My, what a difference a year makes. Last year at this time, Perry announced that she and hubby Russell Brand were splitting. Now, she is coupled up with a fellow musician and has moved on.

Perry and Mayer seem to be a good fit because of their musical nature. Earlier this month, they were snapped taking in a Rolling Stones gig in New Jersey.